As is known, there are three types of solutions for performing a handover of the traffic from one gateway to another:                a handover of the first type, called unsynchronized handover,        a handover of the second type, called “break before make handover”, and        a handover of the third type, called “make before break handover”.        
The unsynchronized solutions of the first type are those which are conventionally implemented in maintenance operations where a backup gateway will act as active nominal gateway for the period of its maintenance. This operation is not finely synchronized between the on board and ground segments, that is to say between the satellite and the terrestrial component of the space telecommunication system, and results in the disconnection of all of the terminals, users of the telecommunication service, with a very significant impact on service.
The solutions of the second type, that is to say those called “break before make” consist in stopping the current access link on the first nominal gateway, then in performing an on board switchover, that is to say at the satellite level, between two access links to the satellite, before activating the second redundant gateway. This type of solution has the advantage of being simple to implement but involves a significant service outage since the operations of this type of handover are performed successively in series.
The solutions of the third type, that is to say those called “make before break” consist in starting up the service on the second redundant gateway as quickly as possible before the on board switchover of the access links. This type of solution involves significant data packet losses in the first nominal gateway, specifically the packets which are in the buffer memories at the moment of the on board switchover, and losses on the new access link because of the loss of synchronization of the user terminals of the satellite system after the on board switchover while a new synchronization for each of the terminals is being re-established.
The technical problem resolved by the invention is how to propose a method for soft handover of data traffic between a first nominal satellite gateway and a second redundant satellite gateway of a space telecommunication system which minimizes the impact on the telecommunication service by reducing the outage times of the gateway-satellite link and/or by avoiding a fine synchronization between the first nominal gateway and the second redundant gateway, thus reducing the data losses in the buffer memories of said gateways and the additional latency introduced, and/or by reducing the time needed to resynchronize the terminals while minimizing the losses of data which could be induced by the user terminals.